A Stirling engine is characterized by having an external heat source as contrasted with an internal combustion engine. The external heat source can come from the combustion of fossil fuels, concentrated solar energy, heat from the decay of radioactive isotopes, hot exhaust gasses from diesel engines, or any other source of heat. Early Stirling engines used air, but modern ones use a gas such as Helium at high pressures to both improve performance and reduce engine physical size.
There are two main methods of transmitting forces from the Stirling power piston to perform useful mechanical work on a load such as an electrical generator. In a so-called “kinematic” design, a power piston, and a displacer piston, if utilized, are connected to a crankshaft, as in a conventional internal combustion engine. The power piston and, if applicable, the displacer piston turn a load such as a rotary electrical generator. In this case, piston excursion is constrained to limits established by the piston's rigid mechanical connection to the crankshaft.
The second configuration is the so-called “free piston” Stirling engine (“FPSE”) wherein a mechanically unconstrained power piston and displacer fundamentally move in linear simple harmonic motion at a frequency nominally equal to a natural mode determined by piston and displacer masses, various restoring spring rates provided by pneumatic, mechanical or other means, and damping effects occurring during engine operation. Typically, FPSE piston displacement is controlled by an appropriate dynamic balancing of input heat flux and mechanical loading to avoid excursions beyond design limits which would cause undesired impact with the cylinder ends. In one typical FPSE application, the power piston is connected by a rigid rod to a cylindrical magnetic structure (often called a “mover”) which cooperates with the fixed portion of a linear electrical alternator. The back and forth movement of the mover/power piston generates an AC voltage at the output of the alternator.
In some applications, the FPSE configuration is preferred to its kinematic alternative, one distinct advantage being that the FPSE virtually eliminates piston-cylinder wall normal forces avoiding the need to lubricate these surfaces and means to isolate lubricant-intolerant engine components.
A cross sectional view of a generic FPSE/linear alternator (FPSE/LA) combination 10 is illustrated in FIG. 1 with the FPSE portion 50 to the left of the figure and the alternator portion 60 to the right of the figure. A gas-tight case 12 contains a freely moving displacer 14 guided by a fixed displacer rod 16. A movable power piston 18 is connected to a permanent magnet structure 20. Various ring seals (not illustrated) may be used to form a gas tight seal between the displacer 14 and power piston 18 and internal part of the case 12. Alternatively, tight radial clearances may be used to limit leakage flows around the pistons and displacer components.
Usually, the four central spaces inside the case are denominated as follows. The space between the displacer 14 and the case 12 is the expansion space 32; the space inside the displacer 14 may serve as a gas spring 34, the space between the displacer 14 and the power piston 18 is the compression space 36; and the space between the power piston 18 and the case 12 is the bounce space 38. The case 12 may be mounted on mechanical springs (not illustrated). Thermal energy to run the Stirling engine is supplied by a heater 40 on the outside of the case 12.
Control of displacer movement both in terms of excursion and its phase relationship to the power piston motion are important factors in FPSE design. In particular, it is advantageous to configure the displacer so that it operates at or near its natural resonant frequency. By enforcing this requirement, many benefits are obtained including engine operation at or near peak efficiency (i.e. for a given input, a higher engine output is obtained).
Prior art solutions generally employ springs of various types in connection with the tuning of displacer movement to a selected resonant frequency based upon particular spring characteristics. Such springs are located within the regions 34 and 36 of the FPSE illustrated in FIG. 1. Typically, in the case of a mechanical spring, the spring is formed as a helical wire and is linked to the displacer 14 and connected between the end of the displacer rod and its cylinder housing.
Natural resonant frequency is a function of both the mass of the collective moving body (displacer and spring) and the spring rate. A given mass-spring system can be tuned to operate at the desired frequency through the control of these two elements in conjunction with the expected damping effect during engine operation. Each particular spring has a single force constant which is determined by its material, geometrical configuration and Hooke's law.
Unfortunately, various drawbacks exist with respect to the use of springs in connection with the control of displacer movement to a particular frequency. Conventional coil springs require the use of a pair of springs deployed in opposition to one another such that the displacer can be controlled in both directions along an axial path. The need for two springs rather than one adds cost and an additional failure point. Another particular problem associated with displacer springs in FPSEs is a less than desirable component life. Prior art mechanical coil springs tend to wear out by flaking, fatiguing and ultimately failing. Various characteristics of prior art spring constructions lead to this result. For example, radially directed and side forces and/or bending moments are applied by the springs upon the displacer and the displacer rod. This can result in decreased wear life both with respect to the spring and with respect to the displacer itself. Further, these side forces increase the static friction between the walls of the displacer rod and the cylinder and can thus also have the effect of impeding initial engine starting.
Additionally, rubbing of the displacer 14 against the containing wall may result if the displacer 14 is not properly centered initially or if it moves off-center as a result of spring changes or spring movement. This is because conventionally coiled spring solutions do not provide any radial stiffness to assist in maintaining the displacer 14 centered on axis.
Another drawback associated with prior art mechanical coil spring solutions is the requirement for a pre-load wherein each of the pair of springs is under some degree of compression at all times even when the displacer 14 is in its rest position. Pre-load is needed to prevent the springs from rattling which, in turn, can cause noise and particulate contamination. Unfortunately, however, pre-loading causes higher stress levels and decreased spring life compared to what could be obtained without a pre-load. Additionally, opposed coil spring designs which are currently in use typically require the use of additional compression space and surface area within the FPSE to accommodate the spring.
Other spring configurations have also been used in connection with displacer control. For example, flat “flexure” or “planar” mechanical spring configurations have been employed in displacer control applications. While these spring configurations typically provide low wear and resulting long life, the mechanical design of the displacer must typically accommodate the unique spring characteristics resulting in more complex displacer design requirements. Additionally, “flat” mechanical spring configurations can be relatively expensive as compared to traditional coiled spring configurations.